In water treatment, a device that uses one or more separation methods out of sedimentation, floatation, and filtration is typically used to separate suspended material from water.
Sedimentation separation is a separation method of separating suspended material from raw water and thus producing treated water by using a density difference between suspended material and water. This method includes the steps of introducing raw water into a sedimentation tank, settling large-sized suspended material that has a higher density than water, and obtaining the supernatant in the sedimentation tank as treated water. In this method, settling speed of suspended material depends on a density or particle size of the material to be separated. Typically, gravity sedimentation or flocculating sedimentation is usually used.
Sedimentation separation is disadvantageous over other separation methods because of low separation speed which is attributable to a long settling time (i.e. about 3 to 6 hours). For this reason, a large sedimentation tank is needed. To increase the settling speed, a plate settler or tube settler is used. However, the use of a plate settler or tube settler adds complexity to equipment or facilities of a water plant and does not sufficiently increase the settling speed. That is, although a plate settler or tube settler is used to increase setting speed, 1 hour of retention time is still required. On the other hand, when the usage of flocculants is increased to improve settling time, a total amount of sediment correspondingly increases. That is, the amount of waste sludge to be disposed increases.
Filtration separation is a separation method of removing suspended material from water by making water pass through a vessel filled with a filter medium, a sieve of close mesh, a filter cloth, or a porous ceramic product. This method is useful for removal of certain material. However, this method has the disadvantages of complicated regeneration work such as backwashing and of high cost attributable to the limited lifespan of filter media.
Flotation separation is a separation method of separating suspended material with a lower density than water by floating the suspended material to the surface of water. This method was first used in ore dressing to separate a metal from ore in the mining industry in 1920s. Then, this method was used to separate pulp in paper manufacturing, used for water treatment in the Republic of South Africa in 1960s, and used later in Scandinavia and other Europe countries.
Since then, the technology of a flotation separation method using dissolved air has been continuously developed in many aspects. As a result, so-called dissolved air flotation, which increases the content of dissolved air in water by pressurizing air, is now a representative solid liquid separation technology. The dissolved air flotation can make material with a higher density than water float to the surface of water along with micro-air bubbles as well as material with a lower density than water. Furthermore, it normally takes only 0.5 to 1 hour to separate and remove suspended material. Therefore, this method is advantageous over a sedimentation separation device in terms of the size of facilities. However, this method also has a disadvantage that a portion of suspended material is resettled on the bottom of a separation tank or transferred to a next process.
In order to solve the problems of the dissolved air flotation method, conventional dissolved air flotation devices that are currently used are equipped with a scrapper driven by a motor on the bottom of a separation tank, thereby collecting settled suspended materials and discharging the collected sludge. However, the addition of a scrapper increases the total cost of equipment, makes operation of equipment more complicated, and cannot entirely remove the sludge. Therefore, it is not effective to remove suspended material that still remains in a separation tank after flotation separation.
A patent document (U.S. Pat. No. 8,349,177) discloses a technology by which: treated water is swirled in a flotation separation tank to increase separation speed of suspended material; two discharge holes are provided to upper and lower portions of the flotation separation tank to discharge scum (froth) and sediments (sludge), respectively; and an umbrella-shaped cover board is installed between a scum discharge pipe and a treated water discharge pipe to prevent floated suspended material from being introduced into a treated water discharge pipe, thereby improving separation performance of suspended material and increasing removal efficiency of sediments. The device disclosed in the patent document has a problem that a scum discharge pipe, a cover board, and a treated water discharge pipe inhibit formation and movement of a swirling flow within a cylindrical separation tank. That is, the device forms turbulent flows in the separation tank, impeding flotation or sedimentation of suspended material. In addition, when water flows within a separation tank, it prevents cross flows in a vertical direction within the separation tank and allows only a descending flow, impeding flotation speed of suspended material and reduces chances of increasing activity of air bubbles in suspended material-air bubble bodies in which suspended material and air bubbles are repeatedly attached to and detached from each other in a dissolved air flotation process.
Dissolved air flotation can remove suspended material using a characteristic that suspended material such as synthetic detergents, softening agents, or proteins gather around the surface of air bubbles or oil when the suspended material exists in water. Since adsorption and removal of suspended material is possible in this way, removal efficiency of suspended material can be increased.